The present invention relates to metal powders and electrolytic capacitors using the metal powders as well as methods of making the powders and electrolytic capacitors.
For many years, it has been the goal of various researchers to develop niobium electrolytic capacitors because of the high di-electric constant of its oxide and the relatively low cost of niobium compared to a variety of other metals. Initially, researchers in this field considered the possibility of using niobium as a substitute for tantalum capacitors. Accordingly, many studies were conducted to determine the suitability of replacing tantalum with niobium.
In some of these studies, however, it was concluded that niobium has serious fundamental deficiencies that needed to be resolved, thus inferring that niobium was not an acceptable substitute for tantalum. (See J. Electrochem. Soc. p. 408 C, December 1977). In another study, one conclusion reached was that the use of niobium in solid electrolytic capacitors seems very unlikely due to various physical and mechanical problems, such as field crystallization. (Electrocomponent Science and Technology, Vol. 1, pp. 27-37 (1974)). Further, in another study, the researchers concluded that anodically formed passive films on niobium were different from electrical properties accomplished with tantalum and that the use of niobium led to complexities which were not present with tantalum. (See Elecrochimica Act., Vol. 40, no. 16, pp. 2623-26 (1995)). Thus, while there was initial hope that niobium might be a suitable replacement for tantalum, the evidence showed that niobium was not capable of replacing tantalum in the electrolytic capacitor market.
Besides tantalum electrolytic capacitors, there is a market for aluminum electrolytic capacitors. However, the aluminum electrolytic capacitors have dramatically different performance characteristics from tantalum electrolytic capacitors.
A driving force in electronic circuitry today is the increasing move toward lower Equivalent Series Resistance (ESR) and Equivalent Series Inductance (ESL). As IC performance increases with submicron geometry, there is a need for lower power supply voltage and noise margin. At the same time, increasing IC speeds require higher power needs. These conflicting requirements create a demand for better power management. This is being accomplished through distributed power supplies which need larger currents for decoupling noise. Increasing IC speeds also mean lower switching times and higher current transients. The electrical circuit must, therefore, also be designed to reduce the transient load response. This broad range of requirements can be met if the circuit has large enough capacitance but low ESR and ESL.
Aluminum capacitors typically provide the largest capacitance of all capacitor types. ESR decreases with increase in capacitance. Therefore, currently a large bank of high capacitance aluminum capacitors are used to meet the above requirements. However, aluminum capacitors do not really satisfy the designers"" requirements of low ESR and ESL. Their mechanical construction with liquid electrolyte inherently produce ESR in the 100s of milliohm along with high impedance.
In the past, solvents such as ethanol have been used during a milling process to make metal flakes. The solvent is desirable to provide wet milling of metal to form flakes. It has been discovered that the use of alcohols and other carbon and oxygen containing solvents for wet-milling or wet-grinding of metal powders to form flakes can create a number of problems. In particular, oxygen present in wet-milling solvents can be released from the solvent due to the ease with which Cxe2x80x94O bonds (carbon to oxygen bonds) are broken. As oxygen enters the system from the broken solvent molecules, the oxygen can react or be present with the metal flakes being formed or with stainless steel milling medium and cause impurities in the resulting flakes. It is believed that breakage of Cxe2x80x94O bonds in wet-milling solvents leads to higher levels of carbon and iron contamination in the produced flakes and causes a resultant corrosive environment.
It is desirable to provide a wet-milling solvent which is substantially inert with respect to metal flakes being formed by the wet-milling process and with respect to the milling medium. It is also desirable to provide a wet-milling solvent which does not break down or decompose during wet-milling.
A feature of the present invention is to provide niobium powders having high capacitance and/or low leakage capability.
An additional feature of the present invention involves developing a process to lower manufacturing costs for making and/or refining niobium powders and other metal powders.
A further feature of the present invention is to provide niobium powders and other valve metals, preferably having high surface areas and physical characteristics, which permit the niobium powders and other valve metals to be formed into a capacitor having high capacitance.
Another feature of the present invention is to provide niobium powders and other valve metals which, when formed into capacitors, have a low DC leakage.
An additional feature of the present invention is to provide niobium powders and other valve metals having better impregnation properties.
A feature of the present invention is to provide wet-milled metal flakes.
A further feature of the invention is to provide wet-milled valve metal flakes, for example, flaked niobium and flaked tantalum.
Another feature of the present invention is to provide a wet-milling process which uses an inert wet-milling fluid or solvent that causes little or no contamination of metal flakes formed from the process.
Additional features and advantages of the present invention will be set forth in part in the description which follows, and in part will be apparent from the description, or may be learned by practice of the present invention.
The present invention relates to a method of making metal powder which comprises the step of milling metal powder at high temperatures and in the presence of at least one liquid solvent. This method can be combined with deoxidized treatments.
The present invention also relates to the use of fluorinated fluids or solvents and more preferably perfluorinated solvents during the wet-milling or wet-grinding of metal powders to form flaked metals, for example, niobium, tantalum, or other valve metal flakes. Typically, any amount of milling fluid can be used during the wet-milling process as long as there is a sufficient amount of fluid to permit a wet-milling or wet-grinding of the metal into flake form. Typically, for every three pounds of metal, about xc2xd to about 1 gallon or more of milling fluid is used.
The benefit of using fluorinated fluids and preferably perfluorinated solvents is that the resulting flaked metal will have lower oxygen levels, lower carbon levels, and/or lower iron levels then comparable flakes made using conventional alcohol milling fluids. Such lower levels of carbon and iron are desirable, particularly in valve metal flakes, since the lower the carbon level and the lower the iron level the lower the DC leakage of the resulting flaked valve metal when formed into a capacitor anode and the higher the capacitance when the flaked valve metal is formed into a capacitor anode. Further, the fluorinated fluids or solvents used in the milling process of the present invention will typically have Cxe2x80x94F bonds as opposed to Cxe2x80x94O bonds as in the case of ethanol, and therefore, less oxygen will enter the milling process when the fluorinated fluid is used. Furthermore, the Cxe2x80x94F bonds of the fluorinated fluids used according to the present invention are quite strong and should remain in tact during the wet-milling process.
In addition, the present invention relates to a process to reduce contaminants involving the step of milling niobium powder or other metal powders at high temperatures and in the presence of at least one liquid solvent. Also, the present invention relates to methods to increase capacitance and/or decrease DC leakage in a capacitor anode involving the steps of milling niobium powder or other valve metals at high temperatures and in the presence of at least one liquid solvent and then forming the niobium powder or other valve metal into a capacitor anode.
The present invention further relates to a niobium powder having a carbon content of from about 40 ppm to about 200 ppm and an iron, nickel, and chromium content of from about 5 ppm to about 200 ppm.
In addition, the present invention relates to a niobium powder having a carbon amount of from about 40 to about 200 ppm when the BET surface area is about 1.0 m2/g and a carbon amount of less than 250 ppm when the BET surface area of the niobium powder is from about 2 to about 4.5 m2/g and the combined amount of Fe/Ni/Cr is less than 100 ppm when the BET surface area of the niobium powder is about 1.0 m2/g and less than about 400 ppm when the BET surface area of the niobium powder is from about 2.0 m2/g to about 4.5 m2/g.
The present invention also relates to methods to reduce DC leakage in a capacitor involving milling niobium chips at elevated temperatures and in the presence of at least one liquid solvent to form niobium powder and forming said niobium powder into a capacitor anode.
Further, the present invention relates to a method of reducing the milling time to form high surface area niobium powder which involves milling niobium chips at elevated temperatures and in the presence of at least one liquid solvent to form niobium powder.
The present invention further relates to the application of the above-described methods for metal powders including valve metal powders as well as the resulting metal powders.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed.